1. Technical Field
The present invention relates to an image forming apparatus and an image forming method.
2. Related Art
To obtain high quality images over a long duration even when printed at high speed in a large amount, it is necessary to use an electrophotographic photoreceptor exhibiting superior electric potential characteristics especially when repeatedly used at high speed and a toner which is fixable at low temperature and still resulting in high quality images, and in addition, it is also essential that the developer containing a electrophotographic photoreceptor and a toner does not deteriorate even after being allowed to stand in an image forming apparatus over an extended period of time.
Commercially available, electrophotographic image forming apparatuses comprise a charger, an image exposure device, a developing device and a transfer device, and an optional cleaning device.
A corona charger is the most popular as a charger generally used, which has the advantage that charging can be stably conducted at high speed. However, application of high voltage to the corona charger produces a large amount of ionized oxygen, ozone, moisture, nitrogen oxide compounds and the like. Such products give rise to problems such as deterioration of an electrophotographic photoreceptor (hereinafter, also denoted simply as a photoreceptor) or adversely affecting the human body.
Recently, there has been studied a contact charging system instead of employing a corona charger. Specifically, a voltage is applied to a charging member such as a charging roller, a charging blade, a charging brush or a magnetic brush, which is brought into contact with a photoreceptor to charge the photoreceptor surface at a prescribed electric potential. The use of such a contact charging system results in reduced voltage, as compared to a noncontact charging system employing a corona charger, thereby leading to reduced generation of ozone. However, there is a further problem in the contact charging system in that a charging roller or the like directly is in contact with a photoreceptor and often abrades the photoreceptor surface or its continuous contact causes peeling-off or cracking of the photosensitive layer of the photoreceptor, easily giving rise to image defects. Specifically, such problems easily occur under severe conditions such high temperature and high humidity (for example, an environment of 30° C. and 80% RH).
Further, in image formation employing color copiers or color printers, an image forming apparatus in which the respective single color images are formed on plural photoreceptors and transferred to a transfer material to form color images, has been noted from the viewpoint of prints being obtained at high speed.
In particular, a photoreceptor containing a charge transporting compound having a triphenylamine structure exhibits superior high-speed responsibility and other satisfactory characteristics.
However, when high-volume printing was conducted under high-temperature and high-humidity, using the foregoing photoreceptor containing a charge transporting compound having a triphenylamine structure in combination with a polymerization toner which has recently been noted as a toner resulting in high quality images, there were produced problems such that image defects occurred. In an attempt to confirm circumstances, it was revealed that a number of cracks were produced in the photosensitive layer of a photoreceptor and specifically when allowed to stand in a stand-by state over a long period of time (e.g., one week) after a large-volume printing run, occurrence of cracking was marked in the photoreceptor at portions in contact with integral members such as a charger, a transfer device and a cleaning blade.
As a result of studying the causes thereof, it was proved that among volatile ingredients contained in a toner, a large amount of volatile aromatic compounds adversely affected the photoreceptor. Specifically, it was found that the volatile aromatic compounds vaporized in the thermal fixing stage and adhered to the photosensitive layer, causing cracks forming image defects at portions in close contact with the charger, a transfer device, a cleaning blade or the like, or the volatile aromatic compounds bled out of the toner, staining the photosensitive layer and causing the cracking.
It was contemplated that occurrence of cracking was marked specifically when the photosensitive layer contained a charge transporting compound having a structure exhibiting affinity to volatile aromatic compounds contained in the toner.
It was also assumed that, specifically in current printers, copiers and color printers, air-exhaustion was suppressed along with miniaturization and gases which vaporized from a toner at the time of thermal fixing, easily accumulated inside the device, and thereby problems became more and more obvious.
A group of toners, called a polymerization toner whereby high quality imaged can be easily obtained, is characterized in that the tone formation of from monomer polymerization to toner particle formation is undergone without allowing resin to stand in a dried state. Accordingly, used raw materials (e.g., solvent, monomer), impurities contained in the raw materials and reaction by-products are easily included in the toner. As a result, such raw materials, impurities contained in the raw materials and reaction by-products are easily filled in an image forming apparatus during storage over a long period of time or at the time of thermal fixing.
When the load applied by a member contacting a photoreceptor is small, excessive stress is not applied to the photoreceptor and cracking rarely occurs. On the contrary, excessively applying the load of a charging roller or a cleaning blade to maintain charging conditions or cleaning conditions results in markedly increased occurrence.
The portion exhibiting cracks causes a gradual increase in potential of the exposure area, readily causing partial fogging.
In the present status, when a high-sensitive photoreceptor using a charge transporting compound, having a triphenylamine structure and a toner exhibiting superior low temperature fixability and achieving high quality image formation are loaded into an image forming apparatus in which a photoreceptor is directly in contact with a charger, a transfer device or a cleaning blade and subjected to printing at a high-volume (e.g., 10,000 sheets) under high-temperature and high-humidity (e.g., 30° C., 80% RH), and thereafter, allowed to stand under high-temperature and high-humidity over a long period of time (e.g., one week), the photosensitive layer of the photoreceptor deteriorates (for example, an increase in potential in exposed areas and occurrence of cracking), forming image defects, whereby superior, high-quality images cannot be obtained over a long period of time.